1. Field of the Invention
The present invention relates to ceramic composites and, in particular, to integrally woven ceramic composite structures used in insulation.
2. Description of the Related Art
In certain high temperature operating environments, such as exterior surfaces of space reentry vehicles and combustion chambers and nozzles in jet engines, rocket engines, and power generators, for example, thermal barriers are necessary to protect supporting structures and equipment. A combustion chamber liner, for example, must be mounted on a strong surrounding structure, typically metal, which must be kept relatively cool and protected from heat, both radiant and conductive. Ceramic materials have utility as thermal barriers because of their high temperature stability. Moreover, since thermal barrier components typically comprise large panels or shell structures that are difficult to fabricate from monolithic ceramics, fiber reinforced ceramic composites are preferred.
In extremely high heat flux environments, such as in rocket engines, the thermal barrier material must also be actively cooled by some mechanism, such as an internally circulating fluid, because the operating temperatures exceed the capabilities of the exposed ceramic materials. All non-ablative rocket nozzles are currently designed in this manner, using high conductivity metals with internal channels for flow of coolant (usually high pressure fuel). The high conductivity is needed to maintain the temperature of the hot surface below the melting point of the metal. If ceramic composites could be used instead of than metals for such structures, large improvements in engine performance would result from: (I) reduced weight; and (ii) reduced heat flux absorbed at the hot surface because of the higher temperature capability of the ceramic. However conventional ceramic composite fabrication methods cannot produce structures capable of satisfying the combined requirements of high pressure containment and high heat flux management.
State-of-the-art ceramic composites are built up to the required thickness using stacked layers of fiber fabrics that are subsequently infiltrated with a ceramic matrix. Unfortunately, such layered composites are not suited to the formation of the structures needed for actively cooled panels because of their susceptibility to delamination of the layers, leading to catastrophic failure.
A preferred approach for forming such panels would be to begin with an integrally woven 3-dimensional fiber preform of the desired shape, with reinforcing fibers in walls and face sheets surrounding internal cavities aligned everywhere predominantly parallel to the stresses expected in use, and to infiltrate the preform with the desired ceramic matrix. Several methods are known for forming integrally woven structures consisting of face sheets connected by walls aligned along the weft direction during weaving. These walls form internal channels which could be used for coolant flow. However, such weft channel structures have several shortcomings for actively cooled structures: (1) the packing density of fiber yarns aligned around the circumference of weft channels (as needed for pressure containment) is inherently limited by the weaving process, so that thicker walls are required to achieve pressure containment, which defeats satisfaction of the heat flux requirements for high performance rocket nozzle and other applications; (2) low packing densities of fibers around channels makes it difficult to achieve hermetic containment of pressurized cooling fluids; (3) the channel lengths in weft channel structures cannot exceed the width of the loom, imposing severe restrictions to structural designs and increasing the difficulty of the weaving process; and (4) weft channel structures are not easily modified to incorporate connecting structures such as manifolds as part of the woven structure at the ends of the channels or elsewhere.
In some systems, passive thermal insulation systems as opposed to active for reentry vehicles is used. One such system includes space vehicle tiles. The passive thermal insulation systems typically comprise very low density ceramic materials bonded to the metal skin of the vehicle. Because of their low density, such materials are very fragile and susceptible to damage from contact with other objects. It would be desirable to provide such low density materials with an outer protective coating of dense tough ceramic composite material that is not susceptible to debonding, or to sandwich it between front and back faces of tough ceramic composite.
State-of-the-art passive thermal protection panels are built up to the required thickness by bonding a low density core of thermally insulating ceramic or other material to a protective skin or thin relatively dense composite sheet consisting of a fibers infiltrated with a ceramic matrix. Unfortunately, such sandwich structures are not durable as thermal barrier panels because of their susceptibility to delamination of the protective skin, leading to catastrophic failure.
A preferred approach for forming such panels would be to begin with an integrally woven 3-dimensional fiber preform of the desired shape, consisting of face sheets connected by walls or struts, the woven reinforcing fibers in the face sheets and walls or struts being infiltrated with a ceramic matrix, and the spaces between the face sheets and walls or struts being infiltrated with a low density insulation material. Several methods are known for forming integrally woven structures consisting of face sheets connected by walls aligned along the weft direction during weaving. These walls form internal channels which could be used for insertion of passive insulation. However, existing channel structures have the severe shortcoming that access to the space between the front and back skins of the structure for inserting passive insulation materials is limited to the ends of the channelsxe2x80x94this is not suitable for the processing methods needed for certain preferred insulation materials.
The present invention comprises an integrally woven 3-dimensional ceramic composite structure with internal channels aligned in the warp weaving direction. The composite includes a multilayer fabric woven from yarns of fibers such as carbon, silicon carbide, silicon nitride, aluminum oxide, mullite, glass, yttrium aluminum garnet (YAG), polyethylene, and other fibrous materials. At least upper and lower layers (or skins) of the composite comprise woven warp and weft yams. The layers may form planes or curved surfaces or tubular structures that can be woven tightly for internal fluid pressure containment. The layers are joined or connected by integrally woven warp and weft yarns forming walls or rows of connecting columns so as to form interior channels in conjunction with the skins.
Weaving processes and designs are chosen in such a way that much higher packing densities of fibers are achieved around the perimeter of each channel to improve the ability of the channels to contain pressure without undue increase in the thickness of either the skins or the walls or columns that form the channel structure.
The woven yarns of the composite material are infiltrated or impregnated with a curing agent that may be in the form of fibers, particulates, powders, vapors, or liquids. The curing agent comprises a material, such as a curable polymer in uncured form or a ceramic precursor, for example, that can be cured by exposure to heat or light (such as infrared or ultraviolet radiation), for example, to form a rigid matrix for the infiltrated fiber yarns. A polymer agent optionally may include ceramic particles so that treatment at higher temperatures will sinter the ceramic particles into a ceramic matrix around the woven yarns and eliminate the polymer or convert it into a ceramic. Ceramic matrix material can also be added after either curing or initial heat treatment by chemical vapor infiltration (CVI) or infiltration of a liquid precursor followed by heat treatment. The resulting structure typically includes two or more layers (skins) connected by walls or struts, in which each of the skins and the walls or struts comprise ceramic reinforcing fibers in a ceramic matrix. The cavities of the open lattice structure can be used for circulation of active cooling fluids (liquids or gases), for example.
A principal object of the invention is a structural ceramic composite that includes utility as a high temperature thermal barrier material. A feature of the invention is a multilayer integrally woven ceramic composite structure with internal channels aligned in the warp weaving direction that can include cooling fluids and can be effectively bonded to a supporting structure. An advantage of the invention is a high packing density of reinforcing fibers aligned in the circumferential direction around the channels to allow containment of high pressure fluid and operation in high heat flux environment. Another advantage is that high packing densities of reinforcing fibers reduce gaps and promote hermetic containment of pressurized cooling fluids. Another advantage of the invention is that there is no limit to the length of channels that can be woven conveniently. Another advantage is that highly curved connecting parts or manifolds can be incorporated in the weave at the ends of the channels or elsewhere.
The above list of advantages may be achieved using either active or passive ceramic composite insulation, or a combination of both. Active ceramic composite insulation includes systems in that a fluid is directed through channels in the insulation. Passive ceramic composite insulation includes systems in that channels in the insulation are filled with randomly packed low density ceramic fibers.
In an aspect of the invention, a woven preform for a ceramic composite comprises a plurality of layers and structural members. The plurality of layers are of woven yarns of fibrous material. The structural members extend between the layers. The layers and the structural members define interlayer spaces. In a further aspect of the invention, a plurality of openings extend through at least one of the layers.
In a further aspect of the invention, low density ceramic insulation is disposed in the interlayer spaces. In a still further aspect of the invention, the low density ceramic insulation comprises fibers having a length shorter than an average width of the openings.
In an aspect of the invention, the plurality of layers of woven yarns comprise an upper layer, a lower layer, and one or more central layers disposed between the upper and lower layers. In an aspect of the invention, at least a portion of the plurality of openings extend through the upper layer. In a further aspect of the invention, at least a portion of the plurality of openings extend through at least one of the central layers.
In an aspect of the invention, the low density ceramic insulation is disposed in the interlayer spaces, the insulation comprising of fibers having a length shorter than an average width of the openings. In a further aspect of the invention, the opening average width is approximately 2 mm or greater. In an aspect of the invention, the low density ceramic insulation comprises Al2O3 fibers or SiO2 fibers that are randomly distributed in a three dimensional arrangement.
In an aspect of the invention, the interlayer spaces are channels. In a further aspect of the invention, wherein the channels extend in a warp direction. In an aspect of the invention, the low density ceramic insulation is disposed in the channels.
In an aspect of the invention, a ceramic composite comprises the woven preform and a matrix.
In an additional aspect, a woven preform for a ceramic composite comprises at least two layers and walls. The two layers are of woven yarns of fibrous material are of woven yarns of fibrous material. The walls extend between the layers. Further, the layers and the walls define channels that extend in a warp direction. In a further aspect of the invention, low density ceramic insulation is disposed in the channels and a plurality of openings extending through one of the layers.
In an additional aspect of the invention, a process for fabricating ceramic composite insulation comprising the steps of providing a woven perform and infiltrating the woven preform. The woven preform comprises a plurality of layers of woven yarns of fibrous material and structural members extending between the layers. The layers and the structural members define interlayer spaces. A plurality of openings extend through at least one the layers. The woven preform is infiltrated with a slurry of a carrier and low density ceramic insulation through the plurality of openings and into the interlayer spaces. At least a portion of the low density ceramic insulation is retained in the interlayer spaces. In an aspect of the invention, a ceramic composite insulation made by the above mentioned process. In a further aspect of the invention, the low density ceramic insulation comprises fibers having a length shorter than an average width of the openings. In a still further aspect of the invention, a ceramic composite that has the short fibers is made according to the above mention process.
In an additional aspect of the invention, a process of insulating a structure comprising the step of joining a ceramic composite to the structure, wherein the ceramic composite comprises a woven preform comprising a plurality of layers of woven yarns of fibrous material and structural members extending between the layers. The layers and the structural members define interlayer spaces. A plurality of openings extend through at least one of the layers. In a further aspect of the invention, fluid is directed through adjacent interlayer spaces. The plurality of layers comprises an upper layer, a lower layer, and at least one central layer disposed between the upper and lower layers. The adjacent interlayer spaces are disposed on opposing sides of a central layer with at least one of the openings extending between the adjacent interlayer spaces, such that the fluid flows between the adjacent channels.
Other aspects, objects, and benefits of the claimed invention are described herein.